Florida produces approximately 5 million tons of orange peel waste each year. Most of this peel waste is dried, pelletized, and sold as beef or milk cattle feed filler commonly referred to as citrus pulp pellets.
High gasoline prices, overdependence on foreign oil, and a continuing demand for renewable energy sources have led to increased research interest in the general field of citrus peel waste conversion, and in particular, to the transformation of peel waste to ethanol. Current processes generally involve hydrolyzing citrus peel comprising a complex mixture of polysaccharides to provide fermentable sugars, fermenting the sugars to produce ethanol, and isolating the ethanol and other by-products.
Unfortunately, some compounds found in citrus peel or produced during the steps converting citrus peel into fermentable sugars act as fermentation inhibitors in the conversion of these sugars to ethanol. Among these compounds, limonene, a terpene-based component in citrus peel, is known to impede fermentation processes (See Grohmann, et al., Production of Ethanol from Enzymatically Hydrolyzed Orange Peel by the Yeast Saccharomyces Cerevisiae, Applied Biochemistry and Biotechnology, Vol. 45 (1994)). Limonene is generally understood to provide a natural defense for citrus against bacteria, viruses, molds, and other organisms and to inhibit fermentation by typical processes that would yield ethanol. It has been estimated that, for efficient fermentation, limonene in the citrus peel waste should be below 3000 parts per million and perhaps even below 1500 ppm.
Stewart et al. (U.S. Patent Application No. 2006/0177916) describes a process of producing ethanol from citrus waste where limonene is removed prior to fermentation. The disclosed process includes limonene removal via evaporation and steam stripping from citrus peel, hydrolysis of the limonene-stripped citrus peel waste, and fermentation of the resulting hydrolysis mixture to produce ethanol (or simultaneous hydrolysis and fermentation). The steam-stripped limonene may be recovered by condensation. Stewart does not address the possibility of employing extraction as a means of reducing the level of fermentation-inhibiting compounds found in citrus waste or the complex polysaccharide or sugars derived therefrom.
Among the problems associated with steam-stripping and similar processes is the substantial usage of energy (e.g., electricity, fuel consumption, etc.) that is necessary to carry out the processes. Such energy expenditures and particularly problematic where the overall goal is to produce alternative fuel sources, such as fossil fuel replacements, because ideally the energy input will be minimized to yield an overall energy-efficient process. Although Stewart discloses the use of centrifuging as an alternative method for removing limonene, Stewart does not address any issues associated with energy usage or otherwise suggest extraction as a means for reducing the level of fermentation-inhibiting compounds.
Previous attempts to use extraction to remove fermentation-inhibiting compounds prior to fermentation have proved unsuccessful. For example, Grohmann (Grohmann, et al., Production of Ethanol from Enzymatically Hydrolyzed Orange Peel by the Yeast Saccharomyces Cerevisiae, Applied Biochemistry and Biotechnology, Vol. 45, 1994) attempted to remove limonene from an aqueous citrus waste mixture using solvent extraction but abandoned the approach because of the formation of inseparable emulsions with a series of organic solvents. Ultimately, Grohmann used filtration to remove the limonene based on the inability to successfully carry out the extraction. Such solutions do not address the problems presented by the extraction and would be problematic on scale-up.
Although extraction has recently been used under certain conditions to remove volatiles from the peel of citrus fruits (Bhupesh, et al., Supercritical Carbon Dioxide Extraction of the Volatiles from the Peel of Japanese Citrus Fruits, J. Essent. Oil Res., 19, 78-84 (2007)), the disclosed procedures did not involve a pre-fermentation extraction step designed to lower the level of fermentation-inhibiting compounds in a feedstock for the production of ethanol. Furthermore, Bhupesh does not teach or suggest converting one or more polysaccharides contained in the citrus waste into fermentable sugars either before or after the extraction, or in turn, fermenting these sugars into ethanol in the absence of the fermentation-inhibiting compounds.
Inasmuch as there is a continuing demand for alternative and/or renewable energy resources, a need to reduce dependence on foreign oil supplies, and a need to reduce or stabilize gasoline prices for example, there is still an unfulfilled need for a specific and effective solution to address one or more of these issues. In view of the above, it is highly desirable to find new methods of eliminating limonene, other similar terpene components, or other fermentation-inhibiting compounds from citrus waste in order to enhance the production of ethanol therefrom. The present invention is directed to these, as well as other important ends.